Filtering circuit



Aug. 25, 1936. H; T BUDENBOM 2,052,338

FILTERING CIRCUIT Filed June 29, 1929 2 Sheets-Sheet 1 ATTRNEY Aug. 25, 1936. H T. BUDENBOM 2,052,338

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Patented Aug. 25, 1936 ENT oEFlcE FmTEmNG vcluctn'r.

Horace T`. Budenbom,-'Shor`twlziills, N. J., assig'nor,` i to Bell Telephone Laboratories, Incorporated, New York, N. Y., a Ycorporation of New York 'Application .iu'ne 29, 1929, serial No. 374,741

1 14 claims` w1.' 17a-44) lThis invention relates to frequency selective circuits and more particularly to systems ofcou pled tuned circuits having band selective prop` erties.

The principal object of,` the'invention is to maintain constant the width of a selected band of frequencies `as-the :mean frequency of the band is varied. Y

4UAnother object is the improvementof the selective characteristics of fradio s receivers and of high frequency amplifiers used in connection with radio or other carrier wave receivers.

A further object is the improvement of the ,I coupling of adjustable tuned circuits whereby a constant widthband selectivity is provided by the impedance propertiesof thecoupling means. In the reception of speech or music transmitted by radio it is necessaryefor good qualityv repro-V duction that the selective circuits of` the receiver should be responsive to a bandof frequencies about 10,000c. p'nsf'wide. For proper discrimination between stations, particularly when the carrier Waves are closely spaced in the `frequency scale, it is also necessary that .the band width should not greatly exceed this valueatanytuning adjustment. To meetbcthoi theserequirements it is therefore desirable V,tliat'tlle selective circuits should beadapted tetransrnit a substantially constant width band of frequencies at all points in the tuning range. d 4 Y .It is well known that when two syntonous', tuned circuits are coupled together electrically the system exhibits thefproperty of double resonance, the two resonance frequenciesbeing separated by an interval depending ,on the degree of coupling. When such a system is used asafrequency selec-.- tive transmission network it may be made to exhibit the properties of a. band-pass wave filterby the proper proportioning of the connected terminal impedances, the limitsof the yband coinciding with the two resonance' frequencies. I fthe degree of coupling .is smallso that the resonance frequencies are .clos'e togethenthel band character ofthe transmissionis maintained in part by the resistances of the circuitelements, and is to some extent independent` ofthe' `values ofthe connected terminal impedances; The location of the band in the frequency scalelneov be varied in the well known manner by'simultaneously changing; the tuning of the individual circuits, but when this is done, unless the degree of coupling isV also adjusted by 'an appropriate amount, the width of the b and will not remain constant. Thus, in the caseI 'oftwotuned- 'cicuits 'c'ljnip'led inductively with 'a` fixed" degree" "of coupling, changing the-tuning by adjustment of the-capaci; Vties' causes the `band width to 'increase proportionately to frequency of resonance'. In accordance withY this 'inventionV the -coupling between a pair off cyril'.cnous'` tunedfcir'cuits 'is 's .o

Va l.

arranged that, as the tuning of the circuits is varied, theV degree of coupling also varies by the amount necessary to maintain a substantially constant band width. The variation of the coupling is effected by the use of aplurality of xed impedance elements in the coupling system the elects of which are substantially complementary over a `wide frequency range. Preferably the coupling system comprises a combination of series and shunt impedances connecting the twotuned circuitsbut other arrangements may also be used in which, for example, the several coupling elements disposed ,inra single branch, form a complex impedance the Value Yof which varies with frequency in the desired manner. By the use o f lfixed coupling elements having complementary effects upon the band Width, the shifting of the band, While maintaining its width constant, is effected with a minimum `number of adjustments,

theonly adjustments necessary being those for 2f) changing the tuning of the syntonous circuits.` j g; O f the attached drawings: e Y y ,Fig 41 is -a schematicshowing the general type of circuit of the invention;

Figs.,2 and V3 represent specific circuits'of the invention corresponding to Fig.1;

Fig #l ,is a representation ofthe bandwidth variatlonin; the circuits of Figs. Zand 3; 4 f A:Figliiillustrates a modified form of the invention;

Fig. 6 shows the band variation for the circuit r-.ligsj andf8. show additional forms of the invention; j, Y t Fig. 9v illustrates the band variation corresponding to Figs. Tand 8; and v 1 Q Fi'g10 illustratesone form oftheinvention e applied to a radioreceiver .input circuit. Y

Referring to Fig. l the selective circuit, which islrepresented in a generalized schematic form, is included between the terminals AA and BB and is illustrated as an interstage coupling `cir cuit between two vacuum tube amplifiers V1 and The circuit is symmetrical in its arrangement yandfis made upof substantially pure reactance elements.V It comprises two equal shunt lreacta .n ces. X1, X1', located adjacent the termi-.- `nals AA Zand. BB', respectively, series reactances X2, X2', also equal, and a symmetrical bridged-T lstructure in the'centerV of .the circuit comprising abridging reactance X4, a shunt reactance X5 anditwo equal connectingreactances X3 and X3. These` reactarices constitute vtwo similar tuned circuits together withta compound coupling sys- Item comprising the shunt reactance X5 and the part of the coupling system together with X4 and X5. Due to the coupling, the circuit is resonant at two frequencies and if the coupling is loose these frequencies fall close together giving the circuit a band transmission characteristic.

A three-mesh circuit such as shown in Figk 1` would in general have three resonances but by the proper selection of the reactances and by the use of proper proportions one of the resonances can be eliminated or else placed well outside of which simplifies to Equating this expression to zero gives the following equations for the determination of the resonance frequencies:

The resonance defined by Equation 2 is that of the series'circuit X1, X2, X3, plus twice the shunt coupling reactance X5 and the resonance defined by Equation 3 is that of the combination constituted by the series coupling reactance X4 plus twice the impedance of X3 in parallel with Xi-l-X2. If these combinations are each singly resonant the network will have only two resonances one of which can be independently controlled by the shunt coupling X5 and the other by the series coupling X4. Further,l if X5 is very small the first resonance will be close to the frequency dened by and if X4 is very large the second resonance will likewise vbe close to this frequency. From this it follows that, with the composite coupling system described, loose coupling of the tuned circuits requires the use of a small shunt coupling impedance and a large series coupling impedance.

In the circuit of Fig. 2 the rst, or primary tuned circuit comprises an inductance m-adeup of two portions VL1 and Lz, corresponding to X1 and X2 of Fig. 1, and a variable condenser. C3 corresponding to X3. The secondary tuned circuit vcomprises similar elements L1",.L2' and C3. The shunt coupling is provided by a condenser C4 and the series coupling by another condenser C5. The circuit is symmetrical as in the case of Fig. 1,

:that is, L1= L1' etc., the equality of the tuned circuits being maintained by having the variable condensers C3 and C3' mechanically coupled so that their capacities are equal for all tuning adjustments. Fig. 2 corresponds to the portion of Fig. 1 between the terminals AA and IBB'.

Applying Equations 2 and 3 to the circuit the .resonance frequencies, denoted by ,f1 and fz are found to be as follows:`

should be large compared with C3 and the capacand If the coupling is loose the resonance frequencies ".,will be relatively close together and the circuit ity C4 should be small relatively to C3. Under this 15 condition the resonance frequenciesrare closely approximated -by the equations y2 f. 1 (1 g4 (5) W 2*1/Ca(L1'i -L2) C: The resonance frequencies lie close to the tunin frequency fo defined by f asf Where w denotes 21r times the frequency.

The variation of the band width with the time ing frequency is `shown by the curves of Fig. 4, which relate to aparticular circuitl designed to maintain a band width of about 10,000 c D, s. throughout a tuning range from 500 k. c per secondv to 1500 k. c. per second. Dotted curve 2 corresponds to the first term on the right of Equation 6, this component of the band width being inversely proportional to the frequency. The contribution of the seriescapacity C4 to the band width isv represented by curve 3 corresponding to the second term of Equation' 6, which varles'as the cube of the frequency. The total band width is shown by the full line curve 4. The value di- .55 minishes at first with frequency, but reaches a minimum and then increases again. For a wide range of frequencies, in the ratio of one to three, the width is nearly constant the variationfrom the average valuebeing only i-20 per cent.` The o minimum band width occurs when the frequency fo has. the. value ,fm given byV Formulae 7 and 8 suffice for the design of the coupling system whenv the values-of .the tuning elements are given. Theminimum band width may be given any desired value, preferably between 5000 and 10,(l00'c. p, s. for speech reception, and may be placed at -any desired point in the frequency scale.. Usually it is preferable to have 'Il the minimum band width occur at a frequency somewhere above the middle of the range since the effect of resistance in the impedance elements is to increase the band width somewhat at the higher frequencies.

' At the resonance frequencies the impedance of the circuit at the terminals AA becomes infinite and at frequencies above and below the resonances the impedance falls olf rapidly to a small value. When the circuit is used in the output of a vacuum tube amplifier, as in Fig. 1, it is desirable that the impedance, at frequencies between the resonances, should be at least as great as the internal impedance of the vacuum tube and, at other frequencies, should be relatively low. In this way the eiect of the tube impedance upon the selective characteristic is made negligibly small. To this end the tuning inductance is divided into two parts, the part L1 between the input terminals AA being relatively small and the total inductance being adjusted to provide the desired range of tuning with a convenient size of condenser. The divided inductance also acts as a step-up transformer, but if' the secondary tuning inductance L1', L2', is divided in the same manner there is no over-all voltage transformation in the circuit. When an over-all voltage transformation is desired it can be obtained by suitably changing the relative values of the secondary inductances L1', L2', a maximum step-up being obtained when all of the tuning inductance is included between the terminals BB. Y

The circuit of Fig. 3 is of a converse type to that of Fig. 2. Here the variable elements of the tuned circuits are inductances La and L3' and the coupling is effected by a small shunt induc-4 tance L and a large series inductance L1. Capacities C1 in shunt and C2 in series constitute the other elements of the primary tuned circuit and corresponding capacities C1' and Cz complete the secondary tuned circuit. The tuning inductances L3 and La are mechanically coupled to maintain syntony of the primary and secondary circuits. The band width in this case is and w denotes 21r times frequency as before. The band width variation is the same as for the circuit of Fig. 2. Also, as in the case of Fig. 2, the xed tuning elements are divided to provide a control of the input impedance and of the voltage transformation.

In Figs. 2 and 3 the series and shunt elements of the compound coupling are of the same character, both capacities or both inductances. Fig. 5 illustrates a circuit in which the coupling is of a 'more complex character, the series coupling impedance comprising an anti-resonant combination, L4, C4,` and the shunt coupling impedance a series resonant combination L5, C5. The priv mary circuit comprises a variable capacity C1 and inductances Lz and L3. The secondary circuit comprises similar elements C1', L2', and Lsf of equal magnitudes to the corresponding elements of the primary circuit. The series coupling Aimpedance is connected at one end directly to the junction of L2' and La' and at the other end is coupled toLa by an inductive winding L31. Coils L3 and Lnare of equal inductance and have substantially perfect coupling, the direction of winding of L31 being reversed to give a reversal of the E. M. F. applied to the coupling impedance. The tuning condensers C1 and C1' are mechanically coupled to maintain the syntony of the primary and secondary circuits.

The band width characteristic may be determined by following the procedure outlined in connection with Fig. 1, but due to the resonant character of the coupling impedances, the equations for the resonances of the system are rather complicated and it is therefore simpler to make use of a modified method of analysis. Consider first the case where capacity C4 and inductance La are omitted, the coupling being effected by means of series inductance L1 and shunt capacity C5 alone. For this case, assuming symmetry of the circuit,

the resonance frequencies are limited to two and are determined by simple equations. Their values are found to be 1 C1 f2*21r/ZE 1+65) 1"2L2 2L3+L4 (12) and the band width to be given by Since capacity C1 is the variable by which tuning is elected its value is inversely proportional to f1. The rst ccmponent'of the band width, represented by the first term on the right of Equation 12 is therefore inversely proportional to the tuning frequency and the second component,

is directly proportional to the tuning frequency. The sum of the components has a variation of the same general character as that illustrated by curve 4 of Fig. 4, but modified slightly due to the fact that the increasing component has a linear instead of a cubic variation. The minimum band Width occurs when and ' minimum is disadvantageous on account ofV the natural tendency o-f resistance in the elements to flatten the resonance peaks and so Widen the effective band. This effect may be counteracted by the addition'of the capacity C4 in parallel with the series coupling inductance and, if desired, by the addition of inductance L5 in series with C5. At the resonance of L5 and C5 the shunt coupling is reduced to zero and at Vthe anti-resonance of L4 and C4 the series coupling is reduced to zero. If these resonances occur at some common frequency above the tuning range, the total coupling of the tuned circuits will be zero at this frequency making the band width also zero. Let the common frequency of resonance of the coupling branches be denoted by fs, then at lower frequenciesthe series branch has an effective inductance-equal to Y and the shunt coupling branchphas an effective capacity equal to The band variation expressed by Equation 14 is illustrated by the cur-ves of Fig. 6, which refer to a circuit designed to maintain a 10,000 cycle band through a range from 400 k. c. to 1800 k. c. per second. Curve 'I corresponds to the component due to the series coupling L4, C4, curve 6 to the component due to L5, C5, and curve 8 shows the variation of the total band width. The suppression freqency fs is placed at 2400-k. c. which is fairly well above the operating range. Since the effect of the shunt coupling is small at the higher operating frequencies, the resonating inductance L5 may be omitted without much effect upon the band width characteristic.

Fig. 7 illustrates another type of circuit in which the band width characteristic is modified by suppressing the coupling at a frequency outside the useful tuning range, the suppression in this case occurring at a frequency below the useful range. vThe circuit corresponds to Fig. 2, but is modified in that the coupling capacity C5 is replaced by a II-network comprising equal shunt capacities Cb and Ca and a series impedance constituted by an inductance Lb in parallel with a shunt capacity Cb. At the anti-resonant frequency of Lb and Cb the coupling through the 1I- network is zero and, except for the effect of the series couplingcapa'city C4 the resonances of the system coincide. At frequencies above this Value the combination Lb Cb is effectively capacitive and the II-network therefore acts substantially like a single shunt coup-ling capacity the value of which decreases progressively with frequency. Theband variation due to the ll-network alone is illustrated by curve I of Fig. 9. The band width is zerol at the frequency fsi corresponding toanti-resonance of Lb and Cb, increases to a maximum at a higher frequency, and then falls o ff approximately inversely With the frequency. -The series capacity C4 produces an increasing bandas the frequency increases and may be adjusted to compensate, in part or in whole, the falling off of the band width due to theshunt coupling capacity.

The circuit of Fig. 8 is a combination of the circuits of Figs. and '7, the shunt coupling be- ,ingthe s ame as that of Fig. '7 and the series coupling corresponding tothe series coupling of Fig. 5. The symbols have the same significance asin Figs. 5 and 'l respectively. In this case the -band width falls to zero at two frequencies, one

below the operating range', due to suppression of theshunt coupling, and the other above the operating range, due to suppression of the series coupling. The band width variation is illustrated by the curves of Fig. 9, curve Ii) representing the component due to the shunt coupling, curve Il representing the component due to the series coupling, andl curve l2 representing the totalfband width. By appropriate adjustments of the coupling impedances curve i2 may be made to slope in either direction or may be made substantially flat. The suppression frequencies fn and Jsz should be located well outside the operating range.

The cir-cuits of the invention are not necessarily confined in their'useto the interstage coupling of amplifiers, but may be applied to all purposes for which tuned circuits or wave filters are' used. Fig. l0 shows the application to the input circuit of a radio receiver. The tuned circuit is of the type shown in Fig. 2, the primary inductance L1 being coupled to an antenna I3 through a small coupling coil I4. Preferably the antenna should be aperiodic or else should have its natural periods well outside the tuning range of the selective circuit, and, in addition, should be loosely coupled to the selective circuit so that its impedancevwill have a minimum effect upon the circuit resonances. In this case the band width is denedby the resonance frequencies and the formulae given above may be used for the design of the circuits.

What is claimed is: v

1. A band selective transmission network comprising a pair of syntonous tuned circuits, means for varying the tuning of said circuits simultaneously, and a coupling impedance connecting said circuits serially, said coupling impedance comprising an anti-resonant circuit tuned to a xed frequency outside the tuning range of said syntonous circuits. K

2. A band selective transmission network comprising a pair of syntonous tuned circuits, means for Varying the tuning of said circuits simultaneously, coupling means for said circuits comprising an impedance common to both circuits and an additional coupling circuit connecting said tuned circuits serially, said additional coupling circuit including an inductance element and being inductively coupled to said tuned circuits in opposite senses respectively.

3. A band selective transmission network comprising a pair of syntonous tuned circuits, each comprising an inductance and a variable tuning capacity, means for varying the tuning of said circuits simultaneously, coupling means comprising a capacity common to both of said cir- .cuits and additional coupling means comprising a loop circuit coupling said circuits serially, said loop circuit including an anti-resonant combination of inductance and capacity and being inductively coupled to said tuned circuits in opposite senses respectively.

4. A band selective transmission network comprising a pair of syntonous tuned circuits including variable tuning condensers of equal capacity, said capacities being mechanically coupled for simultaneous tuning of said circuits, coupling means for said circuits comprising an impedance common to both circuits which is capacitive throughout the tuning range of the network and additional inductive coupling means comprising coupled windings phased to aid the capacitive coupling throughout the tuning range, said capacitive coupling and said inductive coupling being proportioned to provide a substantially constant width transmission band throughout the range of frequencies to which the network can be tuned, and said inductive coupling means includingan anti-resonant circuit tuned to a frequency outside and above the tuning range of the network whereby the degree of the inductive coupling is reduced substantially to Zero at said frequency and the tendency of the inductive coupling to increase the band width at the higher tuning frequencies is counteracted.

5. A network in accordance with claim 4 in which the capacitive coupling impedance includes tuning means for reducing its coupling effect to zero at a frequency outside the tuning range of the network.

6. A band selective transmission network comprising a pair of syntonous tuned circuits means for varying the resonance of said circuits simultaneously, capacitive coupling means for said circuits adapted to produce a band width which decreases as the frequency of tuning increases and tuning means included in said capacitive coupling means adapted to reduce the degree of coupling substantially to zero at a frequency outside and below the tuning range of the network.

'7 A band selective network in accordance with claim 6 and additional coupling means comprising a loop circuit inductively coupled to the tuned circuits to aid the capacitive coupling throughout the tuning range, said loop circuit including a circuit anti-resonant at a frequency outside and above the tuning range of the network.

8. A band selective transmission network cornprising a pair of syntonous tuned circuits, m-eans for ranging the resonance of said circuits simultaneously, capacitive coupling-means and inductive coupling means for coupling said circuits, said coupling means being arranged to transfer energy in aiding relation throughout the tuning range of the network and to provide equal degrees of coupling at a frequencyv within the tuning range, means for reducing the capacitive coupling to zero at a frequency outside and below the tuning range of the network and means for reducing the inductive coupling to zero at a frequency outside and above the tuning range.

9. A band selective transmission network comprising a pair of syntonous tuned circuits, means for varying the tuning of said circuits, and a coupling impedance connecting said circuits serially, said coupling impedance comprising an anti-resonant circuit tuned to a xed frequency outside the tuning range of said syntonous circuits.

10. A band selective transmission network comprising a pair of syntonous tuned circuits, means for varying the tuning of said circuits, and a selective link circuit coupling the variable tuned circuits and tuned to a frequency outside the tuning range of the networks, whereby the coupling effect of the link circuit is reduced substantially to zero at the frequency to which the link circuit is tuned.

11. A network in accordance with claim 10 in which the selective link circuit is composed of xed impedance elements.

12. A band selective transmission network comprising a pair of syntonous tuned circuits, means for varying the tuning of said circuits, and a xed coupling impedance common to said circuits, said coupling impedance being tuned to a frequency outside the tuning range of the network for reducing the coupling effect substantially to zero at the frequency to which the coupling impedance is tuned.

13. A band selective transmission network comprising a pair of syntonous tuned circuits, means for varying the tuning of said circuits and a fixed shunt coupling impedance connecting said circuits, said coupling impedance being tuned to a frequency outside the tuning range of the network for reducing the coupling effect substantially to zero at the frequency to which the coupling impedance is tuned.

14. A band selective transmission network comprising a pair of syntonous tuned circuits, means for varying the tuning of said circuits and a series resonant branch common to said syntonous tuned circuits said series resonant branch being composed of xed elements and resonant to a frequency outside the tuning range of the network.

HORACE T. BUDENBOM.V 

